Electroluminescent display matrix with long persistence



Sept. 26, 1967 R. A. MARTEL 3,344,279

ELECTROLUMINESCENT DISPLAY MATRIX WITH LONG PERSISTENCE Original Filed Feb. 6, 1964 United States Patent 3,344,279 ELECTROLUMINESCENT DISPLAY MATRIX WITH LONG PERSISTENCE Richard A. Martel, Sylmar, Calif., assigner to General Precision, Inc., a corporation of Delaware Continuation of application Ser. No. 343,043, Feb. 6, 1964. This application Aug. 4, 1966, Ser. No. 575,213 7 Claims. (Cl. 250-211) This is a continuation application of Ser. No. 343,043, tiled February 6, 1964 and now abandoned entitled Electroluminescent Display and Storage Matrix, by the same inventor, Richard A. Martel.

Electroluminescent image reproducing systems and units are known, and these units usually include a multi-plicity of spaced parallel conductors on one side of a layer of electroluminescent material, and a similar multiplicity of spaced parallel conductors on the other Side of the layer extending perpendicularly to the conductors of the iirst multiplicity.

When a voltage of a particular -value is introduced across a conductor of the rst multiplicity and a conductor of the second multiplicity, the area of the electroluminescent layer at which the two conductors cr-oss one another will luminesce. Therefore, and in a manner known to the art, static or dynamic images can be displayed by the electroluminescent layer from the selective application of video signal potentials to the diierent conductors of the two multiplicities.

Electroluminescent display assemblies of the type described briefly above are presently in widespread use. However, the field of utility of the prior art units of this general type is limited. This is because of the general inability of the prior art units to store a displayed image for any length of time after the termination of the signals actually producing the image.

This storage ability of the display system is most desirable in -many elds -o-f use. For example, in applications where a study of the images produced by instantaneous transient eiects is desired for any particular purpose, it is extremely important for the display system to be capable of retaining the images produced by the instantaneous transient effects for appreciable time intervals.

An important object of the present invention, therefore, is to provide an improved electroluminescent image display system which is constructed to permit the displayed images to be retained for an appreciable time interval after the cessation of the image producing signals.

Copending application, Serial No. 573,756, led August 4, 1966, discloses and claims a laminated cell construction composed of layers of phosphorescent and electroluminescent material.

The display panel described in the copending application is embodied in `a laminated cell construction. The different layers of lamination in the cell are arranged in a manner similarto that of a parallel plate capacitor, having a layer of phosphorescent material and a layer of electroluminescent material sandwiched between parallel electrode plates.

The parallel electrode plates of the assembly of the copending application are composed preferably of a thin transparent film of electrically conductive material, with two layers of material sandwiched therebetween. One of the layers is composed of a phosphorescent material capable -of exhibiting a selected suitable impedance change which exhibits a relatively long impedance decay time after having been excited by electromagnetic wave energy from a selected band of the radiation spectrum. The other, or second layer, is composed of a suitable electroluminescent material.

As used herein, and in the copending application, the term phosphorescent is deiined as any material which will, when excited by electromagnetic wave energy, emit visible light including the near ultra violet, or near infrared, and whose optical emission will persist for a time greater than 108 seconds after removal of the external excitation, and Which has a relatively long impedance decay time or persistence with respect to the persistence of the optical emission.

As yused herein, and in the copending application, the term electroluminescent is delined as meaning Iany solid material which may be excited to luminescence by the application of an electric field thereacross.

It has been found that most phosphorescent materials, when excited by electromagnetic wave energy, will also exhibit au electrical impedance change which persists after removal of the energy. These materials and their electrical impedance characteristics are known. For example, Patent No. 2,990,473, issued to H. P. Kallmann on June 27, 1961, lists many photoconductors which, when activated in ways identical to the activation carried out in making them luminescent, Will 'become phosphorescent and acquire the characteristics of stored latent pliotoconductivity in which a capacitive impedance change produced by incident electromagnetic radiation will persist for a long period after removal of the radiation and the termination of the visible `lumination of the material.

The electroluminescent material of the electroluminescent layer in the above-mentioned cell is excited to luminescence by the application of an alternating current electric potential across the transparent electrodes, so that any resulting current iiow is between the phosphorescent layer and the EL layer. Since the current flows in series through the phosphorescent layer and the EL layer, the brightness of any point on the electroluminescent layer of the cell is proportional to the electrical impedance exhibited at that point by the adjacent phosphorescent layer.

A typical suitable material for the electroluminescent layers referred to in the preceding paragraphs is a copper activated mixture of zinc oxide and zinc sulfide. Ot course, other electroluminescent materials are also suitable. These materials are described, for example, in an article by Destriau in the Philosophical Magazine, volume 38 (1937).

The operation of the image display cell of the copending application is predicated by the property of the phosphorescent material in the phosphorescent layer to exhibit an electrical impedance change upon the incidence of radiation capable of exciting the phosphorescent layer. This electrical impedance change across the phosphorescent layer in the aforementioned cell persists in the material long after any visible emitted light due to the exciting radiation has ceased.

Therefore, when radiations having wavelengths falling in a particular portion of the radiation spectrum are projected in the form of an image onto the phosphorescent layer of the aforementioned cell, the image is displayed on the phosphorescent layer by corresponding light and dark areas thereof. Concurrently, the electrical impedance across the phosphorescent layer is reduced from point to point in a pattern corresponding to the displayed image, and in direct proportion to the light and shade values of the displayed image.

As mentioned above. this electrical impedance pattern of the phosphorescent layer of the cell persists long after the image on the phosphorescent layer is no longer visible to the naked eye. That is,A the decrease in electrical irnpedance across the phosphorescent layer, corresponding to the displayed image, persists for an appreciable time interval.

When an alternating current potential is applied across the transparent electrodes of the cell described in the copending application, as mentioned above, and when an image formed of radiations of selected wavelengths is projected onto the phosphorescent layer of the cell so as to excite the phosphorescent layer, the resulting changes in impedance across the phosphorescent layer from point to point causes, by voltage-divider action, the actual potential applied across corresponding points of the adjacent electroluminescent layer in the cell to have different values, corresponding to the image displayed by the excited phosphorescent layer.

The impedance and potential patterns referred to in the preceding paragraph enables the applied alternating current potential to cause the electroluminescent layer of the cell described in the copending application to glow and display the image projected onto its phosphorescent layer. Moreover, long after the image on the phosphorescent layer has ceased to be visible, the electroluminescent layer continues to display the image, this being due to the persistence of the decreased impedance pattern across the phosphorescent layer long after the visible effects of the excitation have disappeared and the resulting increased potential across the corresponding portion of the electroluminescent panel.

The improved image display system of the present invention provides a display system which effectively combines the cell described in the copending application, with an electroluminescent image reproducer having crossed conductors to provide image storage capabilities to the entire cell.

When alternating current potentials of a particular frequency are selectively applied to the crossed conductors of the electroluminescent display unit having crossed conductors, the electroluminescent material may be selected to emit visible radiation in the green portion of the radiation spectrum. In addition, the phosphorescent layer of the cell described in the above application may be selected so that it does not respond to light in the green portion of the spectrum.

Therefore, so long as the frequency of the alternating current video exciting potentials applied to the crossed conductors of the electroluminescent image reproducer is held at a selected relatively low frequency, there will be no excitation of the phosphorescent layer, should the aforementioned cell be placed in the path of the radiations from the electroluminescent image reproducer.

However, should the frequency of the alternating current video potentials applied to the cross conductors of the electroluminescent image reproducer be raised to a higher value, the electroluminescent material may be selected so that its emission of visible radiation will shift,

for example, from the green portion of the visible spectrum to the blue portion. This shift is in response to the increase in frequency of the alternating current video potentials.

When the blue emission from the electroluminescent image reproducer is suiciently intense and is composed, for example, of energy in the region of 3660-4050 A., the phosphorescent layer of the aforementioned cell will be excited.

Now, upon the cessation of the alternating current video potentials aplied to the cross conductors of the electroluminescent image reroducer, the excited phosphorescent electroluminescent layers of the above-mentioned cell (which is adjacent the image reproducer) will cause the visible image to persist.

A general object of the present invention, therefore, is to provide a solid state display system which responds to applied alternating current video potentials, and which is capable of displaying visible images in response to such signals.

Another object of the invention is to provide such an improved solid state image display system which can be controlled so that the displayed images persist for appreciable time intervals.

Other objects and advantages of the invention will become apparent from a consideration of the following specitication, when the specification is taken in conjunction with the accompanying drawing, in which:

FIGURE 1 is a perspective view illustrating a display unit constructed in accordance with the concepts of the present invention and representing one embodiment of the invention; and

FIGURE 2 is a sectional View of the assembly of FIG- URE l, taken Substantially on the line 2-2 of FIG- URE 1.

The solid state image display system of the invention' as illustrated in FIGURES 1 and 2, includes, for example, a rst transparent substrate 10 which is composed of glass, or other suitable transparent material.

A rst plurality of narrow, spaced apart parallel conductors 12 are disposed as an array on the transparent substrate 10, as shown in FIGURE l. A layer of electroluminescent material 14 is disposed over and between the conductors 12 in intimate engagement therewith.

A second plurality of spaced apart parallel conductors 16 are formed as an array over the electroluminescent layer 14. The conductors of the second layer are also provided with a layer of electroluminescent material 14 disposed over and between the conductors and likewise between the conductors l12 and 16 of the respective layers. The conductors 12 and the conductors 16 extend transversely to one another, and they are at right angles to one another in the illustrated embodiment. The structure hereinbefore described is more fully explained in United States Patent Number 2,996,623.

The electroluminescent member 14 may be a mixture of a suitable phosphor and dielectric material. For example, as mentioned above, the phosphor may be a zinc sulfide activated with copper, lead and chlorine, as understood in the art; and the dielectric may be a ceramic material.

The conductors 12 and 16 are preferably formed of transparent conductive material. These conductors may be formed, for example, of tin chloride, and they may be applied to the assembly by sputtering, or by other .deposition techniques known to the art.

The electroluminescent cell formed by the components described above may be excited by alternating current video potentials derived from an alternating current source 20. The frequency of these potentials may be adjusted by an appropriate frequency control 22. The source 20 produces video potentials which are selectively applied across the different conductors 12 and 16 by appropriate circuitry (not shown) and in a known manner, so as to cause the electroluminescent cell to produce dynamic or static images.

In ea-ch instance, one of the conductors 16 and one of the conductors 12 is selected and a potential of a particular value is impressed thereacross. The portion of the electroluminescent layer 14 lying between the cross-over points of the two selected conductors is thereby caused to luminesce by the applied alternating current potential.

The intensity of the luminescence at any particular instant is dependent upon the corresponding amplitude of the applied alternating current potential. These amplitudes are controlled in known manner so as to enable the cell to produce a visible image.

The electroluminescent material of the layer 14 is selected to emit radiations extending into the green portion of the radiation spectrum for relatively low frequencies of the alternating current video potentials from the voltage source 20. However, when the frequency of these potentials is increased, the radiations emitted by the electroluminescent layer `14 shift, for example, from the green to the blue portion of the radiation spectrum. The radiations in the blue portion of the spectrum have wavelengths extending in the region of, for example, 3660-4050 A.

The cell illustrated in FIGURES 1 and 2 also includes a pair of transparent electrodes 40 and 42. The ltransparent electrode 40 may, for example, be deposited on the opposite face of the glass substrate 10, and the transparent electrode 42 may be deposited on a second transparent substrate 44. A layer 46 of phosphorescent material is disposed adjacent the transparent electrode 40, and a layer 48 of electroluminescent material is disposed adjacent the transparent electrode 42. These two layers may be separated by a barrier layer 50 formed, for example, of barium titanate, and for the reasons explained in the copending application.

A source S2 of alternating current potential introduces an alternating current potential across the electrodes 40 and 42. In a manner described in detail in the copending application, the alternating current potential from the source 52 excites the electroluminescent layer 48 so that the electroluminescent layer luminesces with an intensity determined by the impedance of the phosphorescent layer 46.

The phosphorescent layer 46 responds to the radiations from the electroluminescent layer 14 when these radiations are in the blue portion of the radiation spectrum. However, when the frequency of the alternating current video potentials applied to the electrodes 12 and 16 is such that the electroluminescent layer 14 luminesces in the green range of the spectrum, the phosphorescent layer 46 is not excited, and it introduces sufficient impedance to the source 52 that the electroluminescent layer 48, likewise, is not excited.

During the unexcited operation of the assembly, the dynamic or static images displayed by the electroluminescent layer 14 may be seen through the substrates l44 and 10, and throughthe transparent electrodes 40 and 42 and the layers interposed between those electrodes. The barium titanate layer forms a suitable screen for the displayed images under these conditions. It Will be appreciated that during the unexcited operation, dynamic images may be displayed by the electroluminescent layer 14 in response to alternating current video potentials selectively applied to the electrodes 12 and 16, and these images are not retained or stored in the display system after the potentials are removed.

However, when it is desired to retain a displayed image in the cell, the frequency of the alternating current video potentials applied to the electrodes 12 and 16 is raised, so as to shift the resulting radiations from the electroluminescent layer 14 into the blue portion of the spectrum. The phosphorescent layer 46 responds to the latter radiations, and exhibits a decreased impedance pattern across its area. This permits the source 52 to excite the electroluminescent layer 48.

Also, and as described in the copending case, the decay characteristics of the excited phosphorescent layer 46 occur over a relatively long time interval after the removal of the radiations, so that the image can be displayed with a relatively long persistence, and after the electroluminescent layer 14 is no longer excited and no longer produces an image.

Therefore, with the cell of the present invention, it is possible at any time to change the frequency of the applied signals, and to cause the image displayed by the cell to be stored therein for a relatively long period of time after the termination of the signals.

While a particular embodiment of the invention has been shown and described, modifications may be made, and it is intended in the following claims to cover all such modifications which fall within the scope of the invention.

What is claimed is:

1. An electroluminescent display and storage system comprising:

(a) excitation means for selectively producing discrete images of electromagnetic radiation, said excitation means comprising an electroluminescent material, a first plurality of parallel transparent conductors disposed on one surface of said material, a second plurality of parallel transparent electrical conductors disposed on the other surface of said material in transverse relationship with said first plurality, and means for applying an electrical potential across selected conductors in said first and second plurality to produce luminescence in said material at the crossover points of said selected conductors,

(b) radiation-responsive phosphorescent material positioned to receive the radiation from said excitation means, said radiation-responsive phosphorescent material having latent photoconductivity storage characteristics for producing impedance changes corresponding to the images of radiation from said eX- citation means,

(c) sensing means coupled to said radiation-responsive phosphorescent material for selectively detecting, continuously or intermittently, the impedance changes in said radiation-responsive phosphorescent material during the period of persistence.

2. An electroluminescent display and storage system, as claimed in claim 1, wherein said sensing means comprises: electroluminescent means which luminesce when subjected to an electric field; and means for applying said electric field to both said layer of phosphorescent material and said electroluminescent means, so that an impedance drop in said phosphorescent material directs more in said electric field across said electroluminescent means.

3. An electroluminescent display and storage system, as claimed in claim 1, wherein said sensing means comprises a layer of electroluminescent material in electrical series with said layer of phosphorescent material, a first transparent electrical conductor upon the surface of said electroluminescent layer opposite said phosphorescent layer, a second transparent electrical conductor upon the surface of said phosphorescent layer opposite said electroluminescent layer, and means for applying an alternating potential across said first and said second conductors.

4. A solid state image display system including: a first layer of electroluminescent material, a first multiplicity of conductors mounted on one side of said first layer, a second multiplicity of conductors mounted on the other side of said first layer in transverse relationship with said conductors of said first multiplicity, means for selectively applying alternating current signals across the conductors of said first and second multiplicities to cause said first layer to emit radiations at the cross-over point of said conductors, having positions in the radiation spectrum determined by the frequency of the applied alternating current signals, a layer of phosphorescent material positioned to receive radiations emitted from said first layer of electroluminescent material and capable of being excited by such radiations which lie in a particular range of the radiation spectrum, said layer of phosphorescent material having an electric impedance which varies in accordance with the excitation thereof by such radiations, a second layer of electroluminescent `material electrically coupled in series with said layer of phosphorescent material, electrode means for applying an electric potential across said layer of phosphorescent and second layer of electroluminescent materials to excite said second layer of electroluminescent material and cause said second layer to luminesce with an intensity determined by the impedance of said phosphorescent layer, and means for controlling the frequency of the alternating current signals applied across said conductors of said rst and second multiplicities selectively to cause the radiations from said first layer of electroluminescent material to extend into said particular range of the radiation spectrum at which said layer of phosphorescent material is capable of being excited.

5. The solid state image display system defined in claim 4, in which said layer of phosphorescent material and said second layer of electroluminescent material are positioned adjacent one another and adjacent said first layer of electroluminescent material, and in which said electrode means comprises a pair of transparent electrodes positioned on opposite sides of said layer of phosphorescent material and said second layer of electroluminescent material.

6. The solid state image display system dened in claim 4, and which includes a layer of barium titanate interposed between said layer of phosphorescent material and said second layer of electroluminescent material.

7. A solid state image display system including: a irst layer of electroluminescent material; electrode means coupled to said first layer; means for applying signals to said electrode means to produce luminescence in said first layer so as to cause said rst layer to emit radiations in a range of the radiation spectrum as determined by the frequency of the signals applied to said electrode means; a layer of phosphorescent material positioned to receive radiations emitted by said first electrolurninescent layer, said phosphorescent layer emitting light upon being eX- cited by such radiations in a particular range of the radiation spectrum and having an electrical impedance which varies in accordance with the intensity of such received radiations, Which impedance variations decay at a rate slower than the decay rate of the emitted light upon termination of such received radiations; a second layer of the electroluminescent material electrically coupled in series with said layer of phosphorescent material; and electrode means for applying an electrical potential across said layer of phosphorescent material and said second layer of electroluminescent material to excite said second layer of electroluminescent material and cause said second layer to luminesce with an intensity determined by the impedance of said phosphorescent layer.

References Cited UNITED STATES PATENTS RALPH G. NILSON,

I; D. WALL, Assistant Examiner.

Primary Examiner. 

1. AN ELECTROLUMINESCENT DISPLAY AND STORAGE SYSTEM COMPRISING: (A) EXCITATION MEANS FOR SELECTIVELY PRODUCING DISCRETE IMAGES OF ELECTROMAGNETIC RADIATION, SAID EXCITATION MEANS COMPRISING AN ELECTROLUMINESCENT MATERIAL, A FIRST PLURALITY OF PARALLEL TRANSPARENT CONDUCTORS DISPOSED ON ONE SURFACE OF SAID MATERIAL, A SECOND PLURALITY OF PARALLEL TRANSPARENT ELECTRICAL CONDUCTORS DISPOSED ON THE OTHER SURFACE OF SAID MATERIAL IN TRANSVERSE RELATIONSHIP WITH SAID FIRST PLURALITY, AND MEANS FOR APPLYING AN ELECTRICAL POTENTIAL ACROSS SELECTED CONDUCTORS IN SAID FIRST AND SECOND PLURALITY TO PRODUCE LUMINESCENCE IN SAID MATERIAL AT THE CROSSOVER POINTS OF SAID SELECTED CONDUCTORS, (B) RADIATION-RESPONSIVE PHOSPHORESCENT MATERIAL POSITIONED TO RECEIVE THE RADIATION FROM SAID EXCITATION MEANS, SAID RADIATION-RESPONSIVE PHOSPHORESCENT MATERIAL HAVING LATENT PHOTOCONDUCTIVITY STORAGE CHAR- 